In the deleted levels, HK-47 was supposed to come to an agreement with HK-51 droids, the successors of the HK-50s, so that these droids would destroy G0-T0 during the finale's G0-T0/Remote standoff. Later, The New Essential Guide to Droids reintroduced this particular cutscene into canon, although HK-51s were replaced with HK-50s.

(Yes, Wook isn't always reliable, but they actually have a source this time. )

I suppose that G0-T0 could be alive in K3 if the Exile was DS in K2, but that would only add to the confusion.

I don't see how the Exile [or Revan] could be imported as a party member. The whole point of party members are to be supportive roles, less powerful overall than the PC. By the end of K1 & K2, I can safely assume, your PC has insanely high skills, Force, and/or combat skills. Having him/her in your party in K3 would make no sense. Unless he/she loses all of his/her experience from the previous game, he/she would be much more powerful than your PC from the start. It would draw attention away from the K3 PC, who is supposed to be the main character with all the potential.

Maybe they could act as a mentor -- i.e., you start your quest off only after Revan or the Exile has taught you everything they know -- or perhaps the (or one of the) quests in K3 would be finding Revan or the Exile (both of whom disappear at the ends of their respective games).

I voted for HK & TM-34, because those are pretty obvious choices, and Handmaiden, just because the idea of her being the "already Jedi" addition to your party (like Visas in K2) appeals to me.

I voted for those three as well. The droids are obvious, but I picked Brianna mainly because of Mr Chee's remark that she "maybe" joined the (female) Exile in K2. It would be nice for canon, plus she's an interesting character. And if the PC isn't Revan or the Exile (a likely scenario), then Brianna could train the PC like she did in K2, maybe teach new Lightsaber/Force forms.

Revan would be the main PC, Exile, Mandalore, HK-47, T3-M4, Bastila, Jolee, Atton should all be party members Yes I know the issues with some of that and there would be new party members ofcourse. All characters that didnt die would play a role in the game especially Carth and Bao Durr working to help the Republic.